RFR: 8338977: Parallel: Improve heap resizing heuristics [v13]

[Albert Mingkun Yang]

On Sat, 21 Jun 2025 20:56:37 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:
>> 
>>  - review
>>  - Merge branch 'master' into pgc-size-policy
>>  - merge
>>  - version
>>  - Merge branch 'master' into pgc-size-policy
>>  - revert-aliases
>>  - Merge branch 'master' into pgc-size-policy
>>  - merge
>>  - merge-fix
>>  - merge
>>  - ... and 9 more: https://git.openjdk.org/jdk/compare/2b94b70e...a21e5363
>
> src/hotspot/share/gc/parallel/parallelArguments.cpp line 71:
> 
>> 69:   // True in product build, since tests using debug build often stress GC
>> 70:   if (FLAG_IS_DEFAULT(UseGCOverheadLimit)) {
>> 71:     FLAG_SET_DEFAULT(UseGCOverheadLimit, trueInProduct);
> 
> Given only Parallel honors `UseGCOverheadLimit`, you may want to set the default to `trueInProduct` in `gc_globals.hpp`

It's indeed that only Parallel implements this flag. However, if we decide to change the default value of this flag globally, I feel we should do that in its own PR for better visibility. Currently, this changes the default value only for Parallel, as one expects in a PR titled "Parallel: ...".

> src/hotspot/share/gc/parallel/parallelScavengeHeap.cpp line 381:
> 
>> 379:   HeapWord* result = young_gen()->expand_and_allocate(size);
>> 380: 
>> 381:   if (result == nullptr && !is_tlab && !should_alloc_in_eden(size)) {
> 
> I feel that you changed intent of `should_alloc_in_eden()`. I believe the original intent is to prevent allocating large objects in eden, and here, seems to prevent allocating small objects in old gen.
> 
> What benefits do you get?

I don't see how I changed the semantics.

On baseline, from `mem_allocate_work`:


if (result != nullptr) {
  return result;
}

// If certain conditions hold, try allocating from the old gen.
if (!is_tlab) {
  result = mem_allocate_old_gen(size);
  if (result != nullptr) {
    return result;
  }
}


and


HeapWord* ParallelScavengeHeap::mem_allocate_old_gen(size_t size) {
  if (!should_alloc_in_eden(size)) {
    // Size is too big for eden.
    return allocate_old_gen_and_record(size);
  }

  return nullptr;
}


The original logic is essentially:


if (result == nullptr && !is_tlab && !should_alloc_in_eden(size)) {
  // allocate in old-gen
}

which is the same as I put here.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2188594599
PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2188604946